Process for making cordierite glass-ceramic having nucleating agent and increased percent cordierite crystallinity

ABSTRACT

A cordierite-type glass-ceramic and a process for its preparation are disclosed, characterized in that a nucleating agent in a small amount is added preferably to a glass batch capable of producing cordierite. The nucleating agent aids in forming a glass-ceramic refractory of relatively large amounts of crystalline cordierite. The melt is quenched and ground to an average particle size no greater than about minus 325 U.S. Standard sieve. This particle size in combination with a relatively small amount of a nucleating agent has been found to increase the amount of cordierite formed during crystallization and simultaneously decrease the amount of glassy phase remaining in the glass-ceramic. In this manner, the ultimate product more nearly resembles pure cordierite. The nucleating agent includes the compounds, usually the oxides, of Mo, Ta, Zr, Nb, Ti, Li, As, and mixtures thereof. The preferred nucleating additive or agent is Nb 2  O 5 . The present refractories may have a coefficient of thermal expansion less than 1.60 × 10 -   6  inch/inch/°C from room temperature to about 600°C as determined by the Orton dilatometer.

BACKGROUND OF THE INVENTION

Cordierite is a crystalline mineral that is desired for its physicalproperties, especially its relatively low rate of thermal expansion.Cordierite occurs naturally in certain rocks, but it is difficult toobtain substantially pure cordierite from this source and virtuallyimpossible on a commercial basis.

Accordingly, the practice has been to synthesize cordierite. Thecrystalline structure of this mineral is generally taken theoreticallyto be a solid solution consisting essentially of the following oxides inthe molar ratio: 2MgO.2Al₂ O₃.nSiO₂, n being a whole number within therange of 5 to 8. As indicated, the cordierite lattice can accept 5 to 8moles of SiO₂ with respect to the ratio given to provide a thermalexpansion only slightly higher than that of the molar ratio 2:2:5,respectively. Beyond the molar ratio of 2:2:8 crystal composition, nofurther silica is accepted and cristobalite inversion appears in theexpansion curves; note "Thermal Expansion of Some Glasses in the SystemMgO--Al₂ O₃ --SiO₂ " by Hummel and Reid, Journal of the American CeramicSociety, Volume 34, No. 10, October, 1951, page 319. Beyond theindicated saturation limit for silica, the presence of free silica isindicated as by changes in the thermal expansion curves.

Although, as indicated, cordierite is considered to have a basicchemical composition, 2MgO.2Al₂ O₃.nSiO₂, there is evidence of limitedsolid solutions in cordierite toward the theoretical compound "Mg-beryl"(3MgO.Al₂ O₃.6SiO₂), and also between cordierite and silica to thecomposition 2MgO.2Al₂ O₃.nSiO₂, where n can be any whole number from 5to 8. The complex nature of the proposed solid solutions and the longtimes that may be required to reach equilibrium in compositions of thistype have resulted in conflicting opinions as to whether these solidsolutions were stable or metastable at room temperature.

The crystal structure of cordierite is similar to the structure of themineral beryl which consists of six-membered silicon-oxygen tetrahedronrings. Because of the differences in chemical composition between beryland cordierite, the six-membered rings in the cordierite structureconsist of one (AlO₄)⁻ ⁵ group and five (SiO₄)⁻ ⁴ groups. Thedifferences between the two forms of cordierite, high or disorderedcordierite and low or ordered cordierite, lie in the degree of order inthe distribution of these aluminum-oxygen tetrahedra in the structure.High cordierite, with hexagonal symmetry, is found in nature as themineral indialite and is most commonly associated with volcanicstructures. Low cordierite has orthorhombic symmetry and is mostcommonly found in metamorphic rocks where the greatest degree ofordering in the cordierite crystal structure is achieved.

Modified cordierite structures also exist including a series of solidsolutions between 2MgO.2Al₂ O₃.5SiO₂ and 2FeO.2Al₂ O₃.5SiO₂, between2MgO.2Al₂ O₃.5SiO₂ and 2MnO.2Al₂ O₃.5SiO₂, and ostensibly between2FeO.2Al₂ O₃.5SiO₂ and 2MnO.2Al₂ O₃.5SiO₂. All these compositionsmaintain the cordierite crystal structure and also may display the sameattractive physical and thermal properties as previously described formagnesian cordierite.

The term "cordierite-type" is used here and in the claims to include anyand all of the cordierite crystalline structures previously described.

Cordierite may be synthesized either through a solid state reaction ofmetal oxides or by recrystallization from a glass. In either case, highcordierite always forms first. Some degrree of structural ordering ofthe alumina-oxygen tetrahedra can be achieved by prolonged heattreatment of high cordierite at elevated temperatures. In the formationof cordierite with the composition 2MgO.2Al₂ O₃.5SiO₂, byrecrystallization from a glass, there are three important reactionswhich occur. An endothermic reaction at about 800°C indicates a pointwhere relaxation in the glass structure occurs, thereby allowing ionicmobility and the formation of nuclei or sites for subsequentcrystallization at higher temperatures. An exothermic reactionrepresenting the formation of beta quartz solid solution crystal phaseoccurs at about 950°C. It is believed that there is a replacement ofpart of the silicon atoms in the crystal lattice of beta quartz byaluminum atoms and the corresponding filling of the spiral-likevacancies with magnesium atoms. The formation of high cordierite at theexpense of beta quartz solid solution and most of the remaining glass isdetected as an exothermic reaction occurring at about 1020°C. Heating totemperatures up to about 1425°C, depending on chemical purity of thestarting glass, ensures maximum cordierite formation for that particularglass composition.

In one process for synthesizing cordierite, the oxides, MgO, Al₂ O₃, andSiO₂, or batch materials yielding these oxides upon firing, such as talcand clay or other aluminum silicates, are smelted and then quenched to asolid glass. A desired article may be shaped from the glass melt priorto a rapid cooling. The glass or preformed article is then reheated to acrystallizing temperature in which cordierite is formed in situ.Unfortunately, the entire mass of solid glass does not convert tocordierite, such that there is coexistence of crystalline cordierite anda glassy phase. The presence of the glassy phase militates againstrealization in the glass-ceramic the desirable properties of cordierite,the more glassy phase being present, the more serious becomes theproblem. While the glassy phase when fluid tends to densify thecordierite, in the resulting glass-ceramic the crystalline cordieriteand vitreous glassy phase compete to dominate its physical properties.Glass-ceramics with the lowest residual glass have also the lowestcoefficients of thermal expansion.

Attempts have, therefore, been made to increase the amount of cordieritethat crystallizes, and thereby reduce the amount of glassy phase formed,by adding nucleating agents to the glass-forming batch. Previously, theamounts added, however, have been relatively large, for example, in thecase of titania of the order of at least 5 to as much as 10% by weightand in some instances as much as 20% based on the weight of the glass.While such additives may have increased the crystallization ofcordierite, their presence in such relatively large quantities has actedas a diluent or poison to the cordierite insofar as realizing thedesired physical properties is concerned. For instance, the rate ofthermal expansion unduly suffers. Use of significant amounts of suchagents may also promote formation of crystalline systems other than thedesired cordierite-type glass-ceramic.

In this respect, prior nucleating agents have a built-in counter-effectwhich at least in part negates any improvement that may have beenrealized in increasing crystallization to cordierite.

SUMMARY OF THE INVENTION

The principal object of the present invention is to provide acordierite-type glass-ceramic having a low rate of thermal expansion forcordierite-glass mixed phases. This is realized by the use of relativelysmall amounts of a nucleating agent in combination with finely groundglass of potential cordierite composition. The combination increases theformation of crystalline cordierite at a crystallizing temperature andthereby decreases the attendant amount of glassy phase formed or leftduring the formation of cordierite.

In the case of bulk glass, crystalliization to a cordierite-typeglass-ceramic initiates at sites largely at the surface and propagatesinwardly. Large crystal sizes and large amounts of residual glassresult. However, when the glass is ground to fine particle size inaccordance with the present invention and then preferably shaped as maybe desired, the same crystallization occurs; but in combination withrelatively small amounts of a nucleating agent and the vastly increasedsurface area, a greatly increased number of nucleating sites are born. Ahigher degree of crystallization to a cordierite-type glass-ceramiceventually results in which the sizes of the crystals are advantageouslylimited by the size of the original glass particles. That residual glasswhich does remain is also appreciably reduced in amount than wouldotherwise be the case.

In one form of the invention, a glass batch capable of yielding a meltconsisting essentially of MgO, Al₂ O₃, and SiO₂ is melted together withup to about 3% by weight of the glass to be formed of a nucleatingagent. The agent is a metal compound, usually the oxide, in which themetal is selected from a class consisting of Mo, Ta, Zr, Ti, Li, As andmixtures thereof. The melt is quenched to a solid glass and the glassthen ground to an average particle size of minus 325 U.S. Standardsieve. The large surface area afforded by the small particle sizesenables the use of a much smaller amount of nucleating agent thanpreviously has been thought possible to achieve the desired results. Infact, especially small amounts, up to 0.5% by weight, have been found tobe especially useful in the case of Ti, Nb, Mo, or Zr. The admixture ofthe glass particles and nucleating agent is formed into a desired shapeand then heated to a nucleating temperature, such as from about 700° toabout 900° C, for a time sufficient to form a plurality ofcrystallization sites, such as about 1 hour to about 4 hours.

The actual role of the nucleating agent is not clear. In some manner,the relatively small amount of nucleating agent is still effective informing many centers or sites of nucleation for forming crystallinecordierite. There results a relatively large number of tiny and randomlyoriented crystals which grow ever larger to a desirable maximum size. Inthis manner, the growth of crystalline cordierite is encouraged with thesimultaneous reduction in the amount of a residual glassy phase. Thecordierite-type glass-ceramic is then formed by heating at a stillhigher, crystallizing temperature, for example, from about 1000° toabout 1200°C for about 1 hour to about 5 hours.

Cordierite-type glass-ceramics of the present invention contain at least80 percent by weight of a cordierite-type crystalline structure and havea coefficient of thermal expansion less than about 1.60 × 10⁻ ⁶inch/inch/° C from room temperature (15° to 30°C) to about 600°C, ascompared to about 2.5 × 10⁻ ⁶ inch/inch/° C usually found incordierite-type bodies, and as compared to about 1 × 10⁻ ⁶ inch/inch/° Cas found in pure polycrystalline cordierite, all as measured on theOrton dilatometer.

The present refractories are adapted for use at high temperatures,thermal shock conditions, such as in heat exchangers, for example, forgas turbine engines, in radome applications, for plates, rods, tubes,and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one form of the invention, batch materials are fused to form a glassmelt containing sufficient amounts of MgO, Al₂ O₃, and SiO₂ to engenderthe cordierite composition. For example, the batch materials may containthe following components or materials adapted to yield such componentsupon smelting: 10 to 30% MgO, 15 to 50% Al₂ O₃, and 35 to 70% SiO₂. Thetheoretical formula 2MgO.2Al₂ O₃.5SiO₂ corresponds to a composition of13.8% MgO, 34.9% Al₂ O₃, and 51.3% SiO₂. Smelting may be carried out at1500° to 1700°C for 1 to 6 hours, although values outside these rangescan also be used.

The nucleating agent may be added at any time prior to reheating theglass that is formed to a nucleating temperature. For example, thenucleating agent may be added to the glass at the time it is ground, ashereinafter described, either before, during, or after the glass isground. In this case, the necleating agent is ground to the same averageparticle size as the glass. However, for convenience and betterdispersion of the nucleating agent throughout the glass, it is preferredto add the agent directly to the original batch materials and to besmelted with them. In any case, a sufficient amount of nucleating agentis added to provide no more than about 3% by weight of the glass. Infact, for certain of the nucleating agents, extremely small amounts havebeen found to be effective. More particularly, for Ti, Nb, Mo or Zr, aslittle as up to 0.5% of the nucleating agent is surprisingly effective.

After the glass melt is formed from the batch materials, it is quenched,preserving the non-crystalline, glassy phase, and the resulting solidglass then conventionally ground to an average particle size of minus325 U.S. Standard sieve (that is, finer than 44 microns) and preferablyup to about five microns. It is postulated that the relatively largesurface area of the particles provide high energy surfaces. The particlesize therefore is significant in the practice of the present invention.The resulting ground glass becomes the raw material for fabricating theceramic product of the invention.

The ground glass particles are formed into a desired article shape bystandard techniques, such as dry pressing, casting, extruding, injectionmolding, or other powder fabrication techniques. In a subsequent heatingoperation to a nucleating temperature, the nucleating agent incombination with the fine particle size promote an increase in number ofnucleating sites for more efficient nucleation.

The ultimate crystal size is ideally limited by the size of the glassparticles and also by the large number of crystals which growsimultaneously. It is thought that the nucleating agents encourage theformation of more sites for crystallization, thereby resulting ingreater crystallinity. The times and temperatures for the nucleatingstep can be developed by trial and error. However, heating for about 1hour to about 4 hours in a range of about 700° to about 900°C has beenfound to be satisfactory. The nucleation at this stage may largely be onthe surfaces of the glass particles. It is believed that many nucleationsites become available, resulting in an overall finer crystal size inthe refractory body.

Thereafter, the article shape can be heated at a still highertemperature in a suitable furnace such as a muffle furnace to promotecrystallization to the cordierite-type refractory. For instance, thesecond heat may be for about 0.5 hour to about 6 hours with a range ofabout 1050° to about 1425°C. Both the nucleating and crystallizing stepsare a function of time and temperature. Refractory products of thepresent invention contain at least 80% by weight of a cordierite-typecrystalline structure. The glass-ceramic may then be allowed to coolwith the furnace.

The original chemical composition of the batch materials may vary widelyas is understood in the art. The batch materials may comprise variousmaterials, either oxides or other compounds which on being heated areconverted to the desired oxide composition. In place of the oxides,other relatively easily decomposed compounds of magnesium, aluminum, andsilica can be used such as the carbonates, sulfates, nitrates, silicicacid, and the like.

In a like manner, the metal compound defining the nucleating agent canvary in chemical composition as long as it is one convertible by heat tothe oxide as during smelting or, additionally, in the case of lithium,in the form of the fluoride. Thus, the term "metal compound" as usedherein to designate the necleating agent includes, as a startingadditive, the metal in free metal form, or its oxide, or a formdecomposable to the oxide or additionally in the case of lithium,lithium fluoride.

In general, following the procedure just outlined provides products ofthe present invention having the following typical properties:

    Modulus of Rupture                                                                         2,000 to 25,000 psi                                              Coefficient of                                                                             Less than 1.60 × 10.sup.-.sup.6 inch/inch/°C,       Thermal Expansion                                                                          from 0°C to 600°C as determined by                              the Orton dilatometer                                            Porosity     5% to 40%                                                        Cordierite-type                                                               Crystalline Content                                                                        At least 80% by weight                                       

At least the metal of the nucleating agent may remain in the ultimaterefractory, normally as the oxide or additionally, in the case oflithium, as the fluoride.

The following examples are intended only to illustrate the invention andshould not be construed as imposing limitations on the claims.Compositions are by weight percent and temperature is on the centigradescale unless otherwise indicated.

EXAMPLES 1 THROUGH 9

Using commercially available raw materials, nine different batchcompositions as shown in Table A were prepared and melted in platinumcrucibles. Calcined magnesite was used as the source of the MgO, whilecalcined alumina was used as a source of the Al₂ O₃. Pennsylvaniapowdered glass sand was used to provide the SiO₂, and reagent gradechemicals were used as a source of the nucleating agents which wereincluded directly in the batch formulations.

All glasses were water quenched to guard against possibledevitrification which could occur on a slow quench. Where necessary, allresulting glasses were crushed and remelted until no unmelted materialwas visible. The quenched glasses were then ball milled to an averageparticle size no greater than about minus 325 U.S. Standard sieve. Eachcomposition of Table A was prepared and processed in the same manner.Press mixes were prepared by adding to a predetermined amount of glassparticles about 4% by weight of polyethylene glycol dissolved indenatured ethyl alcohol. After drying, the samples were passed through a42 mesh screen and about 1.5 weight percent of oleic acid was added andthoroughly blended with the press mixes. A bar sample was then preparedfrom each composition under test by pressing 15 grams of the glassparticles at 5,000 psi in a die of square cross-section measuring 10centimeters in length and 1 centimeter on a side.

The heat treatment schedule consisted of two hold temperatures, namely,one for nucleation, and the other for crystallization. A standard heattreatment schedule had a heating rate of 100°C per hour to a temperaturein the range of 815° to 835°C, followed by a 1-hour hold. Heating wasthen continued to 950°C at a 25°C rise per hour (sintering can occur atthis time) and from that temperature to 1150°C over a 2-hour period,where the samples were then held for 5 hours.

                                      TABLE A                                     __________________________________________________________________________    PHYSICAL, THERMAL AND STRUCTURAL PROPERTIES OF TEST SAMPLES                                                                CTE   CTE   Relative             Batch Formulations    Agent      %     Flexural                                                                            RT-600°C                                                                     RT-600°C                                                                     Cordierite           Example                                                                             MgO  Al.sub.2 O.sub.3                                                                    SiO.sub.2                                                                          Type  Wt.% Shrinkage                                                                           Strength                                                                            Vertical                                                                            Orton Content              __________________________________________________________________________    1     13.8 34.9  51.4 --    --   1.8   2440  1.35  1.60  98                   2     13.8 34.9  51.4 TiO.sub.2                                                                           1.5  2.4   2530  1.23  1.46  100                  3     13.8 34.9  51.4 LiF   0.5  4.2   4940  1.39  --    98                   4     13.8 34.9  51.4 Cr.sub.2 O.sub.3                                                                    1.0  6.3   5970  2.93  3.21  55                   5     13.8 34.9  51.4 Nb.sub.2 O.sub.5                                                                    1.0  3.7   4370  1.32  1.46  97                   6     13.8 34.9  51.4 ZrO.sub.2                                                                           1.0  4.5   6140  1.30  1.54  95                   7     13.8 34.9  51.4 MoO.sub.3                                                                           1.0  5.4   5960  1.30  1.39  97                   8     13.8 34.9  51.4 TiO.sub.2                                                                           6.0  2.6   1550  6.50  --    19                   9     13.8 34.9  51.4 TiO.sub.2                                                                           10.0 10.0  2480  3.61  --    44                   __________________________________________________________________________

Firing shrinkage, rate of thermal expansion and flexural strength weredetermined for each example. Firing shrinkage was determined bymeasuring the percent change in length. Bulk density and porosity weredetermined by the ASTM boiling water method C 20. The coefficient ofthermal expansion (CTE) was determined over a temperature range of roomtemperature (RT) to 600°C on both a vertical dilatometer with a vitreoussilica push rod and sample holder at a heating rate of 160°C per hour,and on a horizontal Orton automatic recording dilatometer, also with avitreous silica push rod and sample holder at a heating rate of 160°Cper hour. The Orton determination of the coefficient of thermalexpansion is more reliable than the vertical dilatometer determination,but the former is also much more time consuming. Use of both techniqueswas made in collecting the data, the vertical dilatometer being employedmore as a screening device. Flexural strength data of Table A weredetermined on an Instron tester by a three-point loading on a 2.5 inchspan.

The cordierite content was determined by standard X-ray diffractiontechnique in which the intensity of peaks was compared with a known,standard material. Crystalline substances such as cordierite exhibit adistinctive group of X-ray diffraction peaks in which the angle thateach peak occurs is dictated by the internal structure of the mineral.The intensity of these peaks is proportional to the amount of thecrystalline phase present in the sample. When a standard is used, theamount of a crystal phase in a sample can be calculated using peakintensities in relation to the intensities of the standard peaks. It isnecessary therefore to use standards of known crystal phase content. Ithas not been possible to obtain a cordierite standard where the absolutecordierite content is known. This is because, if the sample is preparedby recrystallization of a glass having a chemical composition close tothat of cordierite, there is always some residual glass, the amount ofwhich is not accurately known. Samples having the highest amount ofcordierite are either described herein or have been obtained byprolonged heat treatment or both. Extended heat treatments, however,also result in the initiation of the structural inversion from thehexagonal crystal structure to the orthorhombic crystal structure. Thischange in structure results in changes in the angles where the X-raydiffraction peaks occur and also in changes in the intensities of thosepeaks.

The cordierite content as used herein was, therefore, determined bycomparing the cordierite X-ray diffraction peak intensities of a groupof experimental samples and assigning a value of 100 to that sample withan intense cordierite peak, namely, that of Example 2. The compositionsof other examples subsequently analyzed were, in turn, compared to thisexample. These values, then, indicate the relative amounts of cordieriteby weight as detected by X-ray diffraction. However, all data toindicate that all cordierite-type glass-ceramics prepared in accordancewith the present invention did contain at least 80% by weight ofcordierite as 2MgO.2Al₂ O₃.nSiO₂, where n is a whole number in the rangeof 5 to 8.

As shown by Table A, within the amounts of nucleating agent that arecontemplated by the present invention, there were minor differences inshrinkage, strength, coefficients of thermal expansion, and cordieritecontent, with each formulation showing slightly different trends. All ofthe nucleating agents of the invention provided excellent coefficientsof thermal expansion of less than 1.60 × 10⁻ ⁶ inch/inch/° C on theOrton dilatometer. It will be noted that the coefficient of thermalexpansion for Example 1, which contained no nucleating agent was 1.60 ×10⁻ ⁶ inch/inch/° C from room temperautre (RT) to 600°C on thehorizontal (Orton) dilatometer. All of the other formulations containinga nucleating agent of the present invention at amounts less than 3%provided coefficients of thermal expansion less than 1.6 × 10⁻ ⁶inch/inch/° C over the same temperature range as determined by the Ortondilatometer.

Example 4 is included to show the adverse effect of adding a metalcompound not in accordance with the invention. In this case, an additionof 1% by weight of Cr₂ O₃ results in coefficients of thermal expansionof 2.93 × 10⁻ ⁶ inch/inch/° C by the vertical dilatometer; and 3.21 ×10⁻ ⁶ inch/inch/° C by the Orton dilatometer.

Examples 8 and 9 are included to show the effect of adding more than 3%by weight of a nucleating agent. These examples are representative ofthe amounts of added agents used by the prior art. In Example 8, theaddition of 6% TiO₂ resulted in a coefficient of thermal expansion of6.50 × 10⁻ ⁶ inch/inch/° C by the vertical dilatometer. In Example 9,the addition of 10% TiO₂ resulted in a coefficient of thermal expansionof 3.61 × 10⁻ ⁶ inch/inch/° C by the vertical dilatometer.

EXAMPLES 10 THROUGH 21

Table B provides batch formulations for 12 additional examples and thetest results on the glass-ceramics prepared from the formulations.

These formulations were smelted together with the indicated nucleatingagent and test samples prepared from the resulting glasses in the samemanner as described for Examples 1 through 9. In Table B, the valuesindicated correspond to those described for Table A. It will be notedthat the values reported are within desirable ranges and especially thecoefficients of thermal expansion which are less than 1.60 × 10⁻ ⁶inch/inch/° C as determined by Orton and vertical dilatometers.

Although the foregoing describes several embodiments of the presentinvention, it is understood that the present invention may be practicedin still other forms within the scope of the following claims.

                                      TABLE B                                     __________________________________________________________________________    PHYSICAL, THERMAL AND STRUCTURAL PROPERTIES OF TEST SAMPLES                                                                CTE   CTE   Relative             Batch Formulations    Agent      %     Flexural                                                                            RT-600°C                                                                     RT-600°C                                                                     Cordierite           Example                                                                             MgO  Al.sub.2 O.sub.3                                                                    SiO.sub.2                                                                          Type  Wt.% Shrinkage                                                                           Strength                                                                            Vertical                                                                            Orton Content              __________________________________________________________________________    10    13.8 34.9  51.4 TiO.sub.2                                                                           2.5  1.5    830  1.11  1.11  105                  11    13.8 34.9  51.4 TiO.sub.2                                                                           0.8  6.0   5240  1.02  1.18  113                  12    13.8 34.9  51.4 MoO.sub.3                                                                           0.5  3.0   3900  1.03  1.29  107                  13    13.8 34.9  51.4 MoO.sub.3                                                                           2.0  4.5   4560  1.23  --    105                  14    13.8 34.9  51.4 Nb.sub.2 O.sub.5                                                                    0.5  5.0   4560  1.04  --    111                  15    13.8 34.9  51.4 Nb.sub.2 O.sub.5                                                                    1.0  3.0   3790  1.03  1.13  108                  16    13.8 34.9  51.4 Nb.sub.2 O.sub.5                                                                    2.0  2.5   3110  1.03  --     95                  17    13.8 34.9  51.4 ZrO.sub.2                                                                           0.5  7.0   5940  1.20  --    108                  18    13.8 34.9  51.4 ZrO.sub.2                                                                           2.0  10.0  7550  1.41  --     93                  19    13.8 34.9  51.4 LiF   1.0  5.0   3970  1.22  --    112                  20    13.8 34.9  51.4 Ta.sub.2 O.sub.5                                                                    1.0  6.1   3000  1.16  --    102                  21    13.8 34.9  51.4 As.sub.2 O.sub.3                                                                    1.0  5.8   4500  1.13  --    107                  __________________________________________________________________________

We claim:
 1. In a process for producing a cordierite glassceramic byfusing a mixture of ingredients to form a melt consisting essentially ofMgO, Al₂ O₃, and SiO₂, quenching the melt to a solid glass, and thenheating the solid glass to crystallize such oxides to a cordieritecrystalline structure in the presence of a glassy phase; the improvementof increasing the amount of cordierite crystalline structure formed andreducing the amount of glassy phase obtained, comprising: admixing withsaid mixture or glass at any time prior to such heating step anucleating agent in a relatively small amount of about 0.5% to about 3%,based on the weight of said solid glass, said agent consistingessentially of a metal compound, the metal of said compound beingselected from a class consisting of Mo, Ta, Zr, Nb, Ti, Li, As andmixtures thereof, and said compound of the metal being selected from theclass consisting of the metal itself, an oxide of the metal, a compoundconvertible to the oxide by heat, and lithium fluoride, comminuting saidsolid glass to an average particle size no greater than about minus 325U.S. Standard sieve to provide substantially increased surface area,heating said particles and nucleating agent to a nucleating temperatureand forming a plurality of sites for crystallization of said comminutedglass to a cordierite glass-ceramic by the combination of saidrelatively small amount of nucleating agent and said increased surfacearea, and then heating the particles and agent at a higher temperatureto promote crystallization and form a cordierite glass-ceramiccontaining at least 80% by weight of a cordierite crystalline structurecorresponding to 2MgO.2Al₂ O₃.nSiO₂, n being a whole number within therange of 5 to 8, said glass-ceramic having a coefficient of thermalexpansion less than 1.60 × 10⁻ ⁶ inch/inch/° C from room temperature to600°C as determined by the Orton dilatometer.
 2. The process of claim 1in which said cordierite crystalline structure corresponds to 2MgO.2Al₂O₃.5SiO₂.
 3. The process of claim 1 in which said metal compound is Nb₂O₅.
 4. The process of claim 1 in which said nucleating temperature is inthe range of about 700° to about 900°C, and said time is about 1 hour toabout 4 hours.
 5. The process of claim 1 in which said nucleating agentis added to said ingredients prior to forming said melt.
 6. The processof claim 1 in which said heating at a higher temperature includesheating from about 1050° to about 1425°C for about 0.5 hour to about 6hours.
 7. The process of claim 1 including forming a desired shape fromthe glass particles prior to heating to said nucleating temperature. 8.The process of claim 1 in which the metal of said nucleating agent isTi, Nb, Mo, or Zr, said amount of the nucleating agent is about 0.5% andsaid coefficient of thermal expansion is no greater than 1.54 × 10⁻ ⁶inch/inch/° C from room temperature to 600°C.